P E Carr

Lifetime measurements in 40K

The Doppler-shift attenuation method has been used to measure the lifetimes of 19 levels in 40K and four levels in 40Ar. Limits have also been set on the lifetimes of other levels in 40K. The measurements were made using the 37Cl(α, n)40K and 37Cl(α, p)40Ar reactions. Enriched targets of lead chloride and barium chloride were used. The lifetimes obtained with different stopping materials and different recoil velocities were compared where possible and were found to be consistent. A γγ coincidence experiment was performed to elucidate the decay schemes in 40K and 40Ar. The negative-parity states of 40K have γ-ray decay schemes consistent with simple shell-model calculations for d32−1f72 and d32−1p32 configurations. More information is needed before the 2p-2h and 4p-4h positive-parity states can be classified.

Lifetime measurements of levels of 38Ar

Abstract Lifetimes have been measured for 11 levels of 38 Ar using the Doppler-shift attenuation method with the 35 Cl(α, p) 38 Ar reaction. Limits on the lifetimes of four other levels have also been obtained. Several previously unknown γ-ray decay branching ratios have been measured. The other measured branching ratios agree with earlier data except for the decay of the level at 5513 keV. Assignments of 2 + for the 5156 keV level and 3 − for the 5513 keV level are confirmed. The agreement of the lifetimes of the negative parity states with previous shell-model calculations is unsatisfactory for J π = 3 − levels.

Lifetime measurements in 36Ar and 36Cl using the recoil distance method.

The recoil distance method has been used to measure the mean lifetimes of the 788 keV, 1164 keV, 2519 keV and 2810 keV levels in 36Cl and of the 4178 keV and 5171 keV levels in 36Ar. The measured lifetimes are 30+or-1 ps, 7.1+or-0.5 ps, 2.36+or-0.16 ns, 9.0+or-0.6 ps, less than 8 ps and 127+or-5 ps respectively. The data support a correspondence between the 2519 keV and 2810 keV levels of 36Cl with 5- and 4- states given by the shell model calculations of Erne.

Mean lifetimes of levels in 32P and 32S (populated in 29Si(α, p) and 29Si(α, n) reactions)

Lifetimes of low-lying levels in 32P and 32S have been measured using the Doppler shift attenuation method. The levels were populated via the 29Si( alpha , p)32P and 29Si( alpha , n)32S reactions at alpha particle energies of 8.0 and 9.5 MeV. Seven new lifetimes and two new lifetime limits have been measured in 32P and one new lifetime has been measured in 32S. The principal decay modes of some levels have been measured.

Lifetimes, spins and decay modes of levels below 5.3 MeV in 33S

The reaction 30Si( alpha , n)33S was used to populate levels in 33S at alpha particle energies between 7.00 and 10.00 MeV. Gamma rays in 33S were observed in a Ge(Li)-NaI(Tl) escape-suppressed and pair-escape spectrometer. The Doppler shift attenuation method was used to measure the mean lifetimes of 20 levels and to set limits on the lifetimes of 3 levels in 33S. The analysis of angular distributions of gamma rays together with the measured lifetimes and branching ratios gave gamma ray multipole mixing ratios and spin and parity assignments of 7/2+, (5/2, 9/2)+, (3/2+, 5/2) and (7/2, 11/2)- for the levels at 2971, 4050, 4145 and 4868 keV respectively.

Angular correlation measurements on levels in 29Si above 5 MeV excitation (populated in 26Mg(α,n)29Si reaction)

Angular correlation measurements have been made in 29Si to determine spins and mixing ratios using the reaction 26Mg( alpha ,n)29Si. Previously undetermined mixing ratios have been measured for the following transitions: (5.653 MeV-4.742 MeV), delta =-0.02+or-0.20; (5.813 MeV-3.068 MeV), delta =-0.06+or-0.03. The levels at 7.069, 7.522, 8.331 and 8.476 MeV were found to have mean lifetimes pi <15 fs, those at 8.641 and 8.861 MeV have mean lifetimes tau <40 fs.

Gamma-ray linear polarization measurements in 33S

The reaction 30Si( alpha , n)33S was used to populate levels in 33S at alpha particle energies of 7.7 MeV and 9.8 MeV. Gamma rays in 33S were detected in a Ge(Li)-NaI(Tl) escape-suppressed spectrometer and a three Ge(Li) Compton polarimeter. The analysis of angular distributions and experimental linear polarizations of gamma rays gave spin and parity assignments of 5/2+, 9/2+, 7/2+ and 11/2- for the levels at 2868, 4050, 4096 and 4868 keV respectively.

High spin negative parity states in 29Si associated with oblate deformation (population using 26Mg(α,n) reaction)

Spin and parity assignments of 11/2- and 13/2- have been made for levels at 6.781 MeV and 8.761 MeV respectively in 29Si using angular correlation, gamma ray linear polarization and lifetime measurements. The properties of the levels are consistent with their being high spin members of a Kpi =7/2- rotational band based on the Nilsson orbit (303) with oblate deformation.

A study of the principal decay modes and lifetimes of levels in 29Si below 7.2 MeV

The principal decay modes and lifetimes of levels populated in the reaction 26Mg( alpha ,n)29Si have been studied. Lifetimes were measured by the Doppler shift attenuation method at beam energies of 5.0, 6.75, 8.0 and 9.25 MeV. There is good agreement with previous results and two new lifetimes of 20+or-8 fs for the 5813 keV level and 47+or-14 fs for the 7016 keV level have been measured. Thirteen new lifetime limits have also been obtained. The lifetime limit of the 5653 keV level restricts its spin to 9/2+. This level, and that at 7139 keV, are proposed as the 9/2+ and 11/2+ members of the Kpi =3/2+ rotational band and evidence is presented for the inclusion of the 6781 keV level in the Kpi =7/2+ band.

Lifetime measurements for levesl in 30P between 3.7 and 4.7 MeV excitation energy

The mean lifetimes of nine levels in 30P, between 3.7 and 4.7 MeV excitation energy, have been measured using the Doppler shift attenuation method. A limit has also been placed on the lifetime of one further level. Three of these lifetimes have not previously been measured, these are 40+or-30 fs, 180-50+60 fs and 185+or-15 fs for the 4225, 4298 and 4343 keV levels respectively. A doublet has been found at 4229.9+or-1.3 keV and 4234.5+or-1.8 keV where only a single level had been previously reported. The measured lifetimes give transition strengths which lead to spin assignments of 2+ and 3 for the 3834 and 3927 keV levels respectively.